Little is known about the etiology of individual differences during early adolescence. The purpose of the present proposal is to complete the evaluation of adopted and non-adopted children in the Colorado Adoption Project (CAP) at ages 9, 10, 11 and 12 years of age, on a variety of measures including adjustment, personality, general and specific cognitive abilities, social attitudes, physical growth and other variables. These children have been evaluated previously at ages 1, 2, 3, 4 and 7 in other components of the CAP and are, or will be tested at 13 through 16 years of age in another component. The number of CAP test sessions conducted with CAP subjects at ages 9 through 12 during the proposed continuation will be 327, bringing the total to 2,528 sessions. A novel feature of this continuation is the inclusion of the first 400 pairs of twins who have been tested previously on CAP measures at ages 1, 2, 3, 4, 5 and 7 as part of a combined adoption, sibling and twin study of development. We refer to this twin sample as the Colorado Twin Study (CTS) and will conduct 2,388 test sessions with these subjects during the proposed continuation, bring total twin tests to 2,456. The combination of these two comparable samples of subjects greatly strengthens the scope and power of inference regarding developmental processes in this study. The strengths of each approach to genetic and environmental etiology are combined and the limitations of each separate approach are overcome in this integrated genetically informative design. Because of the unique prospective, longitudinal design of both the adoption and twin study, the size and representativeness of their samples, the low rates of attrition, and, in the case of the adoption study, the demonstrated absence of selective placement, together with the use of many of the same measures in the two studies, and their full integrations at ages 4 and 7 years, this continuation provides the opportunity of establishing a truly landmark developmental study of the genetic and environmental etiology of individual differences during early adolescence. Given the investment in the CAP during the previous two decades, the investment in the CTS during the past decade, and the decline in the numbers of newborns available for adoption, a study of this scope and magnitude is unlikely ever to be undertaken again. It is, therefore, crucial to our research endeavor to take advantage of this unique opportunity.